What signs should I look for if I think my 6 year old might have adhd? My 8 year old is on medicine for it and my 6 year old has some similarities too.

See the doctor. If you are suspecting adhd 0r seeing behavior problems i suggest seeing the doctor.
ADHD evaluation. Speak to your childs pediatrician. Your child's doctor will ask several questions and also likely send you home with a questionaire such as the conners or vanderbilt, the doctor may ask you to have the teacher answer questions also. Your child is 6, sometimes it also takes time to mature and become used to routine school work and at home it helps if you have a routine.
Get an evaluation. I would take him to your pediatrician, explain your problems with his attention and behavior and get a full evaluation for adhd.
Have your child eval. Adhd is a complex disorder and requires a proper evaluation by .There are simple checklists available through your pediatrician or online that could help you determine if your child's symptoms are similar to those of children with adhd.However they are not diagnostic.A proper evaluation is indicated as there are other conditions that could present alongside or with similar symptoms.
Different settings. The most important thing for add or adhd is that the symptoms, inattention and/or hyperactivity, be present in school, home and possibly other places. But there is testing available, so see your provider and request testing from your son's school.